Radio-frequency heating system



April 14, 1953 G.H. BROWN RADIO-FREQUENCY HEATING SYSTEM Filed sept. 1, 1949 @f/@roel 46 Patented Apr. 14, 1953 RADIO-FREQUENCY HEATING SYSTEM George H. Brown, Princeton, N. J., assgnor to Radio Corporation of America, a corporation of Delaware Application September 1, 1949, Serial No. 113,581

i2 Claims. 1

This invention relates to high frequency heating apparatus, and more particularly to a system for coupling to a common pair of dielectric heating electrodes two sources of high frequency energ-y which must be independent of each other.

It is oftentimes desirable, in the process of radio frequency heating of dielectric materials, to apply large amounts of radio frequency power to the applicator electrodes. This is especially desirable in cases where long bar electrodes or a long press is used and it is desired to heat treat a thick material or a large amount of materials quickly. For supplying the required increased power, the coupling of a second generator to the electrodes is desirable. However, this poses a problem since the connection to the electrodes must be made such that while both generators supply power to the electrodes, each generator must be isolated from the other to prevent undesirable interactions.

It is the primary object of my present invention to provide an improved system of high frequency heating which permits connection of two radio frequency generators to a common pair of heating electrodes while maintaining the generators isolated from each other.

In the radio frequency heating of dielectric materials, where a long bar electrode or a long press is used, in view of the electrode length, difficulty is experienced in obtaining uniform heating along the length of the bar electrode or press. This results from the presence of standing waves which occur along the electrodes. To overcome the eifect of the standing waves and to equalize the heating along the electrode, it has been customary to use inductive loading stubs which are placed along the electrode length and serve to equalize the voltage along the length of the electrode (see, for example, the Bierwirth Patent No. 2,308,043, granted on June l2, 1943) It is a further object of my present invention to provide an improved system of high frequency heating which provides substantially uniform heating along long bar electrodes without requiring loading stubs.

These and further objects are achieved in my invention by coupling one oscillator to the opposite ends of a bar electrode so that the voltages applied by it to the two electrode ends are in phase and coupling a second oscillator to the same two ends of the bar electrode so that the voltages applied by it to the two electrode ends are in phase opposition. An effective bridge structure is formed by the transmissionlines coupling the bar electrode ends to the generators,

each generator being placed at opposite diagonals of the bridge and the bar electrode being connected between the remaining two bridge diagonals. The generators are thus effectively isolated from each other. The heating effect generated along the bar electrode is substantially uniform. The frequencies at which the two generators oscillate need not be the same or synchronized.

The novel features of the invention, as well as the invention itself, both as to its organization and method of operation, will best be understood from the following description when read in connection with the accompanying drawings in which similar functioning parts are given the same reference numerals and in which Figures 1 through 4 are schematic diagrams of various embodiments of my invention showing systems for connecting two radio frequency generators to a pair of bar electrodes.

v Referring to `Figure l, there is shown a bar or sealing electrodev I9 and a ground plate l2 which are used to apply radio frequency heat to a dieelectric load I4. A radio frequency generator IG which is grounded on one side and which oscillates at a frequency f1 is coupled to the opposite en ds of the bar electrode l0 which terminate its longer axis. The coupling means is a pair of transmission lines i8, 23. The length of one of the transmission lines i8 between the radio frequency generator I6 and one end of the bar eleo-` trode is )q )q mil' where m is any even integer and A1 is the wavelength at the frequency f1 of the generator. The

length of the other transmission line 2l) between the radio frequency generator and the other end of the bar electrode Il! is )u mi Voltages are therefore applied to the two ends of the electrode l0 from the generator i6 by means of two transmission lines i 8, 20 whose electrical impedance is the same but whose length always differs by a half wavelength. The voltages applied to both ends of the bar electrode l0 are therefore always out of phase.

A radio frequency generator 22 which oscillates at a frequency f2 also has one side of its output grounded and has the other side of its output connected to the Vsame two opposite ends of the oar electrode i to which the generator I6 is cormected. The transmission lines 24, 26 between the generator 22 and the bar electrode l0 are each equal in length to where n is any odd integer and A2 is the wavelength at the frequency f2. Therefore, the voltages which are applied to both ends of the bar electrode lil from the radio frequency generator 22 are applied by means of two transmission line sections 2li, 26 which are always equal in length, and therefore these voltages are-always inphase.

Now

and

ifm/2a sin sin (m2o c2) Where V1. and V2 are the instantaneous voltages applied by the respective Hradio frequency gener ators i5, 22 atany point ,at ,a distance X from the center or the bar electrode towards either end, and E1 and Ez'are the root mean square potentials which occur at that point.

The instantaneous power density obtained at the point :z: from the generator I6 with generator 22 shut down, is

.(3) where is the bar electrode conductivity.

Now, if N12 isv averaged .over one :completeicycle of the voltage at frequency f1, thesecond term of Equation .3 averages to Zero, leaving the irst term unchanged. Thus, the average power kP1 inthe dielectric load I4 at distance X maybe expressed as Ifthe generator I6 is now shut down and .the generator 22 is delivering power, then it `can similarly be shown that P2 (average power in the Adielectric I4 at .distance X) may be expressed as ,P1 and P2 cannot be directly added together without an examination of theirbasic relations. With both generators running., the instantaneous power density total is The average of the third term of Equation 6 comes from the average of the product sin (unt) sin (arzt) sin 'mi sin wztzl/z cos (w1-m)t-1/ cos (en-HHH.

The average of either term is zero over an interval of time. Hence, the average power density is Now, if E1 and E2 are made equal, then the total average power density is Whenkis .close to M (flgfz) we may make use of the relation cos2 0+sin20=1 in Equation 8. Then the total..average power densityzUE12. (9)

In ndng the average of the product sin (mt) sin (mt) to be zero, it was assumed that meins.

then

sin wit sin egizi/2 cos (wr-wzH-l/z'cos (w1-leoni :1/w-1/2 cos (Zent) vThe 'latter term only in this equation averages to lZero and the total average power density in this instance becomes and if E1=E2, Equation A10 becomes The 'sine Vterm in Equation k11 Ais undesirable and can be eliminated by phasing the two voltages 90 degreesapart, in which event the productA sin (mi) sin (wz-i-SJTISDIM sin (-w1t+90) :sin mi cos mt which averages to Azero.

.In the operation of theapparatus cimy .n-

v vention, therefore, the generators l .and.22Y are operated so that. equal voltages areapplied to the bar electrode Hl and thefrequencies f1 and f2 aresubstantially close. If fr'and f2 are thesame frequency, then care must be taken to shift the voltage applied by one of the generators 90 degrees with respect to the voltage applied by the other of the generators in order to eliminate the undesirable cross product term.

2[E,2 @OS2 sin2 (alo-tref sinz w21:

The average -of the rst term of Equation -6is The average of the second term of Equation E22 2 (are. 2 S111 ,a

2mn2 cos sin sin (at) sin (mail (t) Now, assume that Af is the diierence in frequency Vbetween f1 and fz, so that fezfi-i-Af. I havefound that if the total bar electrode length does not exceed one half Wavelength, then er fi may be .asjlarge as 0.1 land a power densityvaria- .tion of only i5 percent is obtained along the then the total press length may be over two wavelengths long and a power density variation of only i5 percent is obtained.

Although standing waves exist along the bar electrode for each generator, they serve to compensate for each other and provide substantially uniform dielectric heating along the bar elec trode. The voltage from either of the generators will not interfere with the other since the transmission lines connecting the generators to the bar electrode ends form a balanced bridge with the generators being positioned at two opposite diagonale and the bar electrode being connected between the remaining two opposite diagonals.

Figure 2 is a schematic diagram of another enibodiment of my invention. A line balance converter 28, such as is shown and claimed in my copending application, Serial Number 573,217,

filed January 17, 1945 for Line Balance Converters, now Patent No. 2,517,968, issued August v8, 1950, is utilized to provide equal out-of-phase voltages at the bar electrode ends from the grounded or unbalanced generator 22. The inphase voltages at the bar electrode ends are provided by the generator I6 which is connected to the ends of the bar electrode by two transmission lines 30, 32 having a wavelength which is an odd multiple of one quarter wavelength at the frequency f1.

The theory of operation of the line balance converter 2S is briefly as follows: Current provided by the generator 22, which is connected to the line balance converter 28 goes up through the inner conductor Eli of an input coaxial line 58. This current cannot iiow down the outside of the outer conductor of the central coaxial line E0 because of the fact that a high impedance is presented by the short-circuited quarter `wave section to such ilow. This section is made a quarter wavelength long at the operating frequency of the generator 22. The current `must therefore iiow down the inside of the outer conductor of the central coaxial line 60 to ground. This induces an equal and opposite current 0n the inner conductor of the coaxial line 62. In

view of the connection of this inner conductor,

by means of equal length transmission line sections 34, 36, to the opposite ends of the bar electrode, equal and oppositely phased voltages are caused to exist at the bar electrode ends. The two generators i6, 22 are isolated from each other by virtue of the bridge formed by the transmission lines.

Figure 3 is a schematic diagram illustrating another method of connecting two generators I6, 22a to the bar electrode l@ so that they are isolated from each other and they provide substantially uniform heating along the bar electrode. The generator I6, oscillating at the frequency f1 is connected to the ends of the bar electrodes through two transmission lines of equal length to provide in-phase voltages at the ends of the bar electrode. These transmission lines are each in length, or an odd multiple of one quarter wavelength at the frequency f1. Out of phase voltages are provided at the ends of the bar electrode by the output stage of the other generator 22a. This output stage 34 is readily recognized as a pair of push-pull connected tubes which are coupled to the primary winding of an output transformer 38. The ends of the center tapped output transformer secondary winding are then coupled to the ends of the bar electrode through two transmission lines 34, 36, each of which is half wavelength long at the frequency f2 of the generator 22a. Isolation of the generators is provided by the balanced bridge formed by the transmission lines used for coupling.

Figure 4 is a schematic diagram of still another and preferred embodiment of my invention which makes use of a wideband, balancing radio frequency network 42 of the type described in detail with reference to Fig. 2 of and claimed in my Patent No. 2,454,907, for a Radio Frequency Network. The generator I6, oscillating at the frequency f1 is coupled at terminal A to an input coaxial line 44. The generator 22, oscillatingat a frequency f2 is coupled to another input coaxial line 45 at the terminal B. The inner conductor of the co-axial line 45 is connected to the outer conductor of the coaxial line 44 at a point 48. A portion of the coaxial line 44 is surrounded by a coaxial sleeve of electrically conductive material. The sleeve 50 is connected at one end to the outer conductor of the coaxial line 44. The end of the coaxial line 44 remote from the terminal A is provided with a slot 52 in its outer conductor extending diametrically across the line and longitudinally down to the point 54. The inner conductor of the coaxial line 44 passes through the length of the slot 52 and is connected to the outer conductor of the coaxial line at the free end and at one side of the slot 52. The ends of the bar electrode are connected to the opposite sides of the slot through two equal length transmission lines 5I and 53.

The slot 52 extends longitudinally down the line section one quarter wavelength at the frequency f1 of operation of generator I6. When current at this frequency is supplied to the coaxial line 44 by the generator I6 coupled to terminal A, the instantaneous current flow is over the inner conductor or' the line to the end of the outer conductor at the side of the slot 52. The two parts of the outer conductor of line 44 which are separated by the slot 52 cooperate to act as a quarter wavelength line short circuited at the point 54. This presents a substantially innite impedance between the two slotted ends of the outer conductor of the line 44 and eiectively prevents any of the current from the inner conductor from iiowing down the outer conductor. Thus a current I ows along the line from terminal A and into one end of the bar electroole.

This current I flowing along the inner conductor of the coaxial line 44 from the terminal A is balanced by an equal and opposite current returning down the inside of the outer conductor of the coaxial line 44. This current flows from the other end of the bar electrode to the upper slotted end of the outer conductor of the line 44. Owing to the high impedance across the open end of the slot 52, all of the returning current flows down inside the upper portion of the outer conductor of the line section 44 below the slot 52. Thus, the opposite ends of the bar elec- Vtrode are energized in phase opposition by the generator coupled to the terminal A.

The cuter conductor ofthe line section fill cooperatesl with the sleeve 59 to function as a coaxial 'line section short circuited at the end near the terminal A. Current from the generator 2g coupled to terminal B is applied to this line section at the point e8. This current induces a voltage along `the outside of the outer conductor of the line section lill by autotransiormer action. The voltage to ground from both arms of the slotted, outer conductor is the same. Thus', the voltage between the slotted ends of the outer conductor is zero, and no current is induced in the coaxial line ed by the generator coupled to the terminali?,

VSince the two ends of the outer conductor oi the line lill are at the saine potential with respect to ground, the voltages at the 'two ends of the bar electrode coupled thereto by the two equal length transmission lines are also at the same potential. These voltages are in phase with each other. None of the current from the terminal B flows on the outside of the outer conductor of that portion of the line :le within sleeve 5G, so no current is induced in the generator coupled tothe terminal A.

The radio freouency network thus provides a means for applying irl-phase voltages from one generator and out-oi-phase voltages from a second generator which are applied to the ends 'ot a bar electrode, and yet the two generators are'isolated from one another because of the bridge actionof the radio frequency network. En my copending application for Radio Transmitters, application Serial Number 52,635, filed October i948, now Patent 2,602,857, issued July 8, i952, there is describedand claimed a system for ieedn ing two transmitters to an absorber element and a common load using the above described radio frequency network. The-rein. it is shown that the output from the two generators is added in a load and neutralized in an rabsorber element. in the instant application, the radio frequency network is used to supply a single load (namely the bar electrode which is connected across its output) simultaneously with in-pliase out oi phase voltages.

From the foregoing description, it will be readily apparent that I have provided an improved system or apparatus for coupling two generators to a common pair of electrodes order to provide substantially uniform heating along the electrode and to maintain the generators isolated from each other. Although several embodiments have been shown and described, it should be apparent that many changes may be made in the several inbodiments herein disclosed, that many other embodiments are possible, all within the spirit and scope of my invention. Thereicre, I desire that the foregoing description shall be taken as illustrative and not as limiting.

I claim as my invention:

l. A system for radio frequency heating comrising a radio frequency source of iii-phase voltages, o. radio frequency source of substantially 180 degree out-of-phase voltages, a bar electrode, and bridge network means coupling said sources to the opposite ends of said bar electrode to provide substantially uniform heating therealong and isolating one of said radio frequency sources from the other.

y2. A system for coupling two sources of radio frequency heating energy to a bar electrode for dielectric heating comprising a balanced bridge circuit having four diagonale, said bar electrode being coupled between `two opposite diagonal's of said bridge circuit, each of said two sources of radio frequency heating energy being coupled to each of the other two opposite diagonale, and degree voltage phase shift means in one Voi the arms of said bridge.

3. A system for coupling two sources of radio freouency heating energy to a bar electrode comprising a balanced bridge circuit having four diagonal's, said bar electrode being coupled between two opposite diagonale of said bridgec-ircuitfone of said two sources of radio frequency' heating energv being coupled to one oi the other diagonale of said bridge to provide in-phase voltages'atfsaid bar electrode opposite ends, and means in said other bridge diagonal coupled to the other of said two sources of radio frequency heating energy to provide substantially degree outo-iphase voltages at said bar electrode oppositeends.

4. The system recited in claim 2 wherein said means insa-id bridge diagonal coupled to the other of said two sourcesof radio frequency energy to provide said out-of-phase voltages at said 'bar electrode oppositeends comprises a push-pull output transformer.

5. rI'he system recited in claim V2 wherein said means in said bridge diagonal coupled to the other of said two sources of radio frequency energy-to provide said out-cf-phase voltages at said'bar electrode opposite ends comprises a line balance converter.

6. A system for coupling two sources of radio frequency heating energy to a bar electrode comprising a iirst pair of transinission lines connected together at one end Aand coupled at said one end. to one of said two sources, the other ends of said first pair oi transmission lines being coupled to the opposite ends of said bar electrode, eachiof said first pair of transmission lines being anodd multiple of a quarter wavelength long at the frequency of operation of said one source to provide in-phase voltages at said bar electrode opposite ends, and a second pair of transmission lines connected together at one end and coupled to the other of said two Sources at said end,'the other ends of said second pair of transmission lines b-eing coupled to said opposite ends of said bar electrodes, one of said second pair of transmission lines being an even multiple of a quarter wavelength long at the frequency of said other of said two sources, the other of said second pair of transmission lines being one half wavelength longer than said one of said second pairof'transmission lines to provide out-of-phase voltages at said bar electrode opposite ends whereby saidtwo sources are isolated from each other and substantially uniform heating is provided along the bar electrode length.

7. A system for coupling two sources of radio frequency heating energy to a bar electrode comprising a iirst pair of transmission lines connected together at one end and coupled at said one end to one of said two sources, the other ends of said iirst pair of transmission lines being coupled to the opposite ends oi said bar electrode, each ci said rst pair or" transmission lines being an odd multiple of a quarter wavelength long at the frequency of operation of said one source to provide iii-phase voltages at said bar electrode opposite ends, a line balance converter for converting an unbalanced input to a balanced output, said other source of radio frequency heating energy being coupled to said line balance converter input, a second pair of transmission lines having a length equal to one half wavelength at the frequency of said other source, said second pair of transmission lines being coupled from said line balance converter output to said bar electrode opposite ends to provide out-of-phase voltages at said bar electrode opposite ends whereby said two sources of radio frequency heating energy are isolated from each other and substantially uniform heating is provided along the bar electrode length.

8. A system for providing substantially uniform radio frequency heating along a bal` electrode comprising a first radio frequency generator, a second radio frequency generator having a pushpull output, a first pair of transmission lines connected together at one end and coupled at said one end to said first radio frequency generator, the other ends of said first pair of transmission lines being coupled to the opposite ends of said bar electrode, each of said first pair of transmission lines being an odd multiple of a quarter wavelength long at the frequency of operation of said first radio frequency generator, and a second pair of transmission lines coupled between the push-pull output of said second radio frequency generator and said bar electrode opposite ends, each of said second pair of transmission lines being one half Wavelength long at the frequency of operation of said second radio frequency generator to provide out-of-phase voltages at said bar electrode opposite ends whereby said rst and second radio frequency generators are isolated from each other and there is provided substantially uniform heating along said bar electrode.

9. A system for providing substantially uniform radio frequency heating along a bar electrode comprising a radio frequency network including a first coaxial input line, a conductive sleeve surrounding one end of said line and connected to the outer conductor of said line at a point near said end, the end portion of the outer conductor of said line including a longitudinal slot, the end of the inner conductor of said line being connected to the end of the outer conductor of said line at one side only of said slot, a second coaxial input line having one conductor connected to said sleeve and the other conductor coupled to the outer conductor of said rst input line within said sleeve, a rst radio frequency generator coupled to said first coaxial input line, and a second radio frequency generator coupled to said second coaxial input line, the opposite ends of said bar electrode being coupled between the two opposite portions of said longitudinal slot at the l0 portion on said line outer conductor where said inner conductor end is connected to said outer conductor end whereby said rst and second radio frequency generators are isolated from each other and uniform heating is provided along said bar electrode.

10. A system for radio frequency heating comprising a first and a second source of radio frequency energy, a bar electrode, and network means isolatingly coupling said rst and second sources to opposite ends of said bar electrode to apply substantially in phase voltages to said bar electrode from said rst source and substantially degree out-of-phase voltages from said second source across said electrode.

11. A system for radio frequency heating comprising a first and a second source of radio frequency energy, a bar electrode, and network means including means coupling said first source of radio frequency energy to opposite ends of said bar electrode to apply substantially in phase voltages across said electrode, and means coupling said second source of radio frequency energy to said opposite bar electrode ends to apply substantially 180 degree out-of-phase voltages across said electrode.

12. A system for radio frequency heating comprising a first and a second source of radio frequency energy, a bar electrode, and coupling means isolatingly connecting said iirst and second sources to opposite ends of said bar electrode, said coupling means including means to apply substantially in phase voltages to said bar electrode ends from said first source, and means to apply substantially 180 degree out-of-phase voltages to said bar electrode ends from said second source.

GEORGE H. BROWN.

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